1. Field of the Invention
The present invention relates to apparatus and methods for demodulating a modulated signal, for example, according to one or more of a π(pi)/4 differential quaternary phase shift keying (DQPSK) modulation scheme, a pi/2 differential-binary phase-shift keying (DBPSK) modulation scheme, a Gaussian minimum shift keying (GMSK) modulation scheme, a Gaussian Frequency-Shift Keying (GFSK) modulation scheme, a Differential M-ary Phase Shift Keying (DMPSK) modulation scheme or an M-ary Phase Shift Keying (MPSK) modulation scheme.
2. Description of Related Art
M-ary Phase Shift Keying (MPSK) and Differential M-ary Phase Shift Keying (DMPSK) are widely used in wireless communication and satellite communication. In MPSK modulation scheme, the phase of the current symbol is switched among M values according to the modulated binary input signal to be transmitted. In DMPSK modulation scheme, the difference between the phase of the current symbol and that of the preceding symbol in a transmitted signal is based on the modulated binary input signal. Both MPSK and DMPSK are linear modulation techniques and the modulated symbols are conventionally pulse shaped by a linear finite impulse response (FIR) filter which commonly uses a squared root raised cosine (RRC) filter or a raised cosine filter (RC). MPSK and DMPSK techniques have many variations such as BPSK, QPSK, 8PSK, DBPSK, DQPSK, pi/2 DBPSK, pi/4 DQPSK and Offset QFSK.
In traditional demodulators for MPSK and DMPSK signals, an analogue to digital converter (ADC) is used to convert the received analogue signal (either a baseband signal or an intermediate frequency (IF) signal) into digital form for further processing. For digital processing, the signal is passed through a matched filter, such as an RRC filter, and is then applied to a timing synchronization stage. For MPSK signals, either a voltage-controlled oscillator (VCO) or a Numerically Controlled Oscillator is used to track the carrier phase and frequency. For DPSK signals, a differential detector which consists of four multipliers is used to extract the phase difference between two consecutive symbols. One drawback of this arrangement is that analogue-to-digital conversion typically consumes a large amount of power. Another drawback is that the Radio Frequency (RF) front end of the receiver requires Automatic Gain Control (AGC). A further disadvantage is that the inclusion of a VCO or an NCO is inherently complex, requiring large and expensive circuit configurations. In such conventional processes, it is also necessary to normalize the received in-phase (I) and quadrature (Q) signals to determine the decision thresholds for phase measurement.
Attempts have been made to circumvent these problems. U.S. Pat. No. 5,574,399 adopts a hard limiter to replace analogue-to-digital conversion and the instantaneous phase of the received PSK signal is detected with reference to the un-modulated carrier signal. The phase rotation due to frequency offset between the two IF signals is detected and removed from the instantaneous phase signal. A remaining phase offset is then detected and removed. Data are recovered from the resulting instantaneous phase signal.
Other patents such as U.S. Pat. Nos. 3,997,847, 5,122,758, 5,539,776, 5,640,427, 5,945,875, 5,574,399 and 5,668,838 also disclose replacing analogue to digital conversion with a hard limiter. In these processes a very high sampling rate is required to deal with the differential detection, whereas, in digital processing systems, a simple correlation and a counter are commonly used for the phase detector. Furthermore, there are a number of other disadvantages in the above-mentioned methods which use a hard limiter, and these include a serious performance degradation relative to other conventional methods in which an analogue to digital conversion takes place. Also, these methods are primarily used for dealing with pi/4 DQPSK modulation which uses little power and is insensitive to the nonlinearity in the RF components. However, in conventional MPSK and DPSK techniques, such as D8PSK modulation, the signal may go through the origin at the constellation, and is much more sensitive to the non-linearity introduced by power amplifier, mixers, narrow band pass IF filter and limiter. Thus, the performance of a D8PSK demodulator could deteriorate and such a demodulator would require good linear characteristics for the RF front end components, especially for strong input signals. Those apparatus using coherent detection require more accurate frequency offset estimation than those using differential detection.
Thus there is a need for apparatus and methods which substantially overcome or at least ameliorate some or all of the abovementioned disadvantages.